1. Field of the Invention
This invention relates to light-emitting diodes and more particularly, a system and method for controlling light-emitting diodes.
2. Description of the Related Art
At the present time light-emitting diodes, denoted LEDs hereafter, are increasingly being used to provide lighting and/or signalling functions for automotive vehicles.
Generally, several LEDs are associated in an optical unit and powered synchronously so as to ensure the aforementioned functions.
When, in particular, a gradual control of the illuminating light intensity, or dimming, of the LEDs must be achieved, the aforementioned groups of LEDs lend themselves satisfactorily to such a control mode, by means of a modulation of the duration, called the width, of the current pulses powering these groups. The modulation of the width of the pulses, voltage pulses converted into current pulses by the internal impedance of the aforementioned LEDs, is obtained, for example, using a DC-to-DC voltage converter, with which a switchable control circuit is associated that is controlled by a gradual control signal that makes it possible consequently to adjust the duty cycle and therefore the width of the current pulses powering the LEDs, and finally the electrical power delivered to the latter and therefore the illuminating light intensity of the groups of LEDs powered in this way.
The aforementioned operating mode is completely satisfactory, at least with regard to pulses having high duty cycles, higher than about 20%, i.e., for average or high LED illumination levels.
In particular, for conventional DC-to-DC converter circuit/switchable control circuit pairs, since the switchable control circuit comprises a proportional-integral loop for controlling the voltage level of the pulses, the limited dynamic, i.e., response time, of the assembly thus arranged means that the power supplied to the LEDs takes too long to settle.
Furthermore, when a gradual control of the illuminating light intensity of the LEDs is applied so as to obtain low lighting level, the duty cycle of the supply pulses then being set to a value lower than a value of about 20%, the excessively slow rise time of the pulses reduces the average value of the current supplied to the LEDs, which average value is no longer equal to the product of the amplitude value of the pulses in the steady state and the duty cycle value used for application of the gradual control. This results in a non-linearity of the gradual control, which causes this control to be less precise. Furthermore, the average value of the supply current of the LEDs depends on the supply voltage.
A solution intended to obviate the aforementioned drawbacks has been provided in patent DE 10236872.
In the aforementioned solution an analogue circuit, having in series a capacitor and a resistor R2, C2, controlled by a switch T2, as shown in FIG. 1, (a reproduction of FIG. 2 of the aforementioned patent), in fact enables the maximum value of the amplitude of the pulses to be sampled and stored for the following pulse.
Such a system is satisfactory technically, but it requires perfect synchronization of the switching of the switch T2 and the switch T3 enabling application of the supply pulses to the LEDs.
Furthermore, such a solution requires the use of a specific switchable control circuit 4 that must withstand the connection of the sampling circuit R2, C2 to the reference voltage, ground, of the device via the switch T2.
This is not the case for conventional switchable control circuits normally available on the market.
What is needed, therefore, is a system and method for controlling LEDs that improves over those of the past.